This invention relates to a new and improved composite molded article and to a method for producing such articles having outer skins, a glass fiber reinforced layer and a core of lower density material containing a filler.
In the development of processes for producing molded articles from polymerized resinous materials, the desirability of increasing the strength by the inclusion of high tensile strength fibers, in particular, glass fiber material, has long been recognized. The molding of such reinforced articles generally involves bringing the resin and resin-reinforcing fibers together, shaping them to a desired configuration, and holding the shape until the resin has cured or set to hold the desired shape permanently.
A common technique for production of reinforced articles involves the use of pre-forms of glass fibers prepared by collecting chopped strands of glass on a screen conforming to the shape of the part. The pre-form is then inserted into a mold and saturated with a polymerizable resin and a foamable material. Cure of the foam and resin produces a rigid article. Such rigid plastic foams have found varied use in industry; for example, large panels can be employed as structural units for wall or ceiling panels, walk-in refrigerators, cold-room liners, all-weather buildings and in recreational vehicles and boats.
If the article to be molded is essentially planar or of little depth or curvature, pre-cut glass mats can be used in place of pre-forms and are placed within the mold as described above or cut to shape and manually fit to the mold. When the molded article is large or when it is desired that the article have a low density, for buoyancy properties, etc., the use of liquid resins containing a low density particulate filler has become widespread, in addition, of course, to the common use of conventional foams in which the liquid resin contains a blowing agent to create voids when cured. Foams of the former type include those known as syntactic foams containing previously formed bubbles of various materials or, more broadly, a particulate filler of lower density than the resin itself.
In many applications, conventional or syntactic foams are used with glass fiber reinforcing layers or chopped fibers in composite articles having a non-foamed outer shell which presents a smooth and blemish free surface. Aside from those processes in which a non-porous skin is formed about a foamed core in a single injection or molding operation, as typified by U.S. Pat. No. 3,268,636, most composite articles are fabricated essentially as a laminate in which a pre-formed, impervious shell of a thermoplastic sheet is filled with the foam material, with the layer or layers of fiber reinforcement positioned somewhere within the foam core, preferably near the outer shell where the reinforcement function is most needed.
For example, U.S. Pat. No. 3,503,841 discloses a sandwich-type structure having at least one outer skin formed from a polyvinyl chloride sheet and a polystyrene foamed core containing short chopped asbestos or glass fibers throughout the core. This approach, using chopped fibers distributed throughout the core, will provide a satisfactory product for many uses but is not as satisfactory from a strength standpoint for large structures as are composites in which the fiber reinforcement is in the form of woven or matted layers positioned near the skin of the composite.
U.S. Pat. No. 3,711,581 discloses a process for fabricating a composite article having pre-formed impervious thermoplastic skins, a foamed polyurethane core and layers of a glass fiber or other fibrous mat adhered to the inner surfaces of the skins. The process specifically provides for adhering the fibrous mat to the skin by a separate application of a bonding agent to the skin's inner surface, then a second separate application of the bonding agent to the fibrous mat before foaming the core. The use of the separate bonding agent on each surface prior to foaming recognizes the importance of obtaining good adhesion between the fibrous mats and the skin necessary for the fibrous layer to perform its reinforcing function for the skin where the highest stresses will occur.
U.S. Pat. No. 3,591,444 discloses a process which endeavors to eliminate the necessity of a separate bonding agent as used in the aforementioned '841 patent. Recognizing the importance of the bond between the outer skin and the fibrous mat, the structure utilizes a random fiber, three-dimensional "fleece" adjacent the inner surface of the conventional woven fiber "pre-preg" such that individual fibers of the fleece extend laterally into the core and in the opposite direction into the woven mat while at the same time, the fleece and mat restrict the foaming action so that, hopefully, a more dense resin-rich marginal area is provided adjacent the outer skin. The use of the pre-impregnated mat adjacent the fleece is necessary to assure adherence of this mat to the outer skin or gel coat, if one is used; otherwise, the pre-preg mat forms the outer skin and is bonded to the core through the laterally extending fleece.
Each of the above summarized processes, recognizing the criticality of the bond between an impervious outer shell and the adjacent fibrous mat, appear to solve the problem with the use of added adhesive layers or additional constituents such as the fleece used in the '444 process. None appear to be suitable for use with syntactic type foams wherein the particulate filler is carried by the moving liquid resin under pressure, as opposed to conventional foams wherein the liquid resin itself is moved by the expanding gaseous bubbles. In particular, it would appear that the function of the fleece in the '444 process above would be destroyed by the particles of filler in a syntactic foam which would push the laterally extending fleece fibers back into the plane of the skin, mat and fleece upon the application of pressure.
It is accordingly an object of this invention to provide a process for producing a composite article having an impervious outer skin adhered to a fibrous mat which surrounds a lower density core wherein the resin of the core material acts as the binder or adhesive between the skin and mat surfaces, to thus eliminate separate adhesive or other elements heretofore necessary to improve this bond.